This invention relates generally to capacitance measuring devices, and, more particularly, to a capacitance measuring device which includes the capability of providing an indication of overrange measurements.
Capacitors are found in most of todays electronic circuits. Therefore, in either analyzing such circuits or building such circuits, it is necessary to actually determine the value of the capacitors utilized therein. In many instances, however, capacitors are impossible to mark. They are frequently of a small physical size, which precludes printing a number of them.
In the past, various forms of bridge-type testing devices have been employed in order to ascertain the value of such capacitors. These devices usually are in the form of a Wheatstone bridge which operates on the principle of comparison or balance of capacitances. Unfortunately, such devices are extremely costly and are also subject to change of values of the elements in the bridge circuit, per se, and therefore require frequent calibration. Another device which has shortcomings, provides capacitance values by measuring the charging or discharging current. Furthermore, difficulties have been encountered when employing either of the above systems which stem largely from the fact that daily calibration is required and special procedures must be followed in operating the bridge network which, for example, requires technical skill and training.
A capacitance meter which overcomes some of the drawbacks associated with past capacitance measuring devices is set forth in U.S. Pat. No. 2,940,037. The invention described therein employs a "free running" multivibrator which generates a wave-form having a predetermined time constant. This generated signal is differentiated by a pulse shaping circuit and then applied to trigger a monostable multivibrator having a micro-ammeter placed in the circuit of the normally "off" side thereof. The monostable multivibrator has a quasi-stable state determined by an unknown capacitor undergoing test and, for a fixed driving frequency, the average value of current through the meter is determined by the duration of the quasi-stable state of the monostable multivibrator. Unfortunately, the capacitance meter of the type set forth above, is incapable of ascertaining whether or not the tested capacitor falls within the preselected range of the capacitance meter. In addition, the particular electronic components chosen for this capacitance meter have inherent drawbacks associated therewith.
Therefore, it is highly desirable to provide a device, capable of ascertaining capacitor values which is inexpensive to manufacture, simplistic in design, portable, self-calibrating and highly accurate in making such capacitance measurements.